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The response of an elliptically polarized antenna to a quasi-monochromatic, partially polarized radio wave is treated from the standpoint of coherence theory. A general formula is derived for the available power at the terminals of a receiving antenna in terms of appropriately chosen coherency matrices for the antenna and the incident wave. It is shown that the result is formally identical with the basic interference law of partially coherent, quasi-monochromatic wave fields. Conditions for the maximum available power are discussed, and a geometrical interpretation of the result is given. The coherency matrix for the antenna is determined uniquely in terms of the transmitting properties of the antenna. It is shown that the coherency matrix formalism for the interaction of an antenna with an incident wave fits very well with those used in the modern theory of optics and quantum mechanics. A new definition for the antenna effective aperture using the coherency matrix is suggested, which takes into account the polarization property of the antenna.